The Stovebolt Site Forums The Shop Area Engine Shop Vapor Bypass Fuel Filter

I have never seen a specific PSI, but I think they bypassed at about 10 PSI. We retrofitted them to older vehicles here in Texas. During hot weather the fuel would expand in the fuel line between the pump and the carb after the engine was turned off. The needle and seat could not handle the pressure and the carb would flood when you tried to start it again. The bypass does not come into play while the engine is running. The fuel pump or an after market pressure regulator regulates the pressure while the engine is running.